An all position automatic welding apparatus includes all devices necessary for welding such as a welding power supply, a welding head having a torch, a control device for controlling the driving of the welding head, etc., a cooling water circulating pump for cooling the torch. In the all position automatic welding apparatus, for example, in the welding head that is a machine for clamping the to-be-welded tubes 22 and moving the welding torch 7 shown in FIG. 10 through FIG. 12, in particular, an automatic TIG welding head makes a welding head having a torch 7 clamp the fixed tubes 22, rotates the torch 7 along the welding portion on the outer circumference of the tube 22 while inserting a wire (hereinafter, may be referred to as a rotating direction), and simultaneously, while adjusting the arc length by moving the torch 7 in the radial direction of the tube 22 on the outer circumference of the fixed tubes 22 (hereinafter, may be referred to as a movement in the vertical direction or just as a vertical movement), whereby carrying out multi-layer welding. Therefore, the welding head needs a mechanism for moving the torch 7.
A typical example of butt welding of tubes 22 by using the all position automatic welding head is welding of waterwall tubes forming a furnace wall of a boiler. As shown in FIG. 14, a number of tubes 22 are arranged in parallel, a membrane bar 44 is disposed between adjacent two tubes 22, 22, and the bar 44 and the tubes 22 are welded, whereby a waterwall tube (boiler panel) of a furnace wall is obtained. A plurality of such waterwall tubes are prepared according to the specifications of the furnace wall structure, these waterwall tubes are assembled so as to form a furnace wall, and the butting portion of the waterwall tubes is welded and continued with the all position automatic welding head A to form a furnace wall.
As shown in FIG. 14, at the welding portion where tubes 22, 22 of the ends of adjacent two waterwall tubes are butted against each other, both tubes 22, 22 are subjected to edge preparation, and the welding head A is attached to the gap between the two tubes 22, 22 including the non-existence of the membrane bar 44, and the butting portion of both tubes 22, 22 is welded with the torch 7, and this butt welding is carried out by repeating the welding one layer at a time to form a multilayer while rotating the portion of the torch 7 of the welding head A along the outer circumferences of the tubes 22.
Namely, as indicated by the operation locus of the torch 7 in the case of multilayer-welding along the outer circumferences of the tubes 22 in FIG. 15, in order to prevent a cooling water hose for cooling the portion of the torch 7 attached to the welding apparatus or welding power supply wire from being repeatedly wound around the welding head A, an arc is cut for each one-layer welding as shown in zigzag line (z) of the outer circumferences of the tubes 22, the torch 7 is rotated in reverse along the outer circumferences of the tubes 22 to return to the welding start position as shown in straight line(s), and then next-layer welding is carried out. Multilayer welding is carried out by repeating this operation, and in this case, to adjust the arc length for each layer welding and adjust the torch height for the next welding, the welding head A is operated to move in the vertical direction of the tubes 22.
When welding the waterwall tube, the gap between adjacent two tubes 22, 22 forming one waterwall tube is a narrow as 17 mm, and it is required to be movable in the above-mentioned vertical direction on the outer circumferential surfaces of the tubes 22 even in this narrow gap.
As a conventional technique of an automatic welding head A having a torch 7 that is movable in the circumferential direction and vertical direction of the tubes 22 in such a narrow gap, inventions are disclosed in Japanese Published Unexamined Patent Application No. H09-271939 and No. 2001-225165.